kotlin, how to add extension function to already defined class companion object

i want to add some extension function to an already defined class‘s companion object, like

fun Toast.Companion.showImageToast(str : String) {
}

and also i like to do the same thing on interfaces, e.g

fun Runnable.Companion.mainHandler() {
}

I've consulted the documentation, only to find syntax for define extension function for companion object in a user defined class, not for class that don't have a companion object

can i have any chance to do this in kotlin ?